1. Field of the Invention
An object of the present invention is a mirror elbow or bend having a directional coupler for very high power microwave transmission lines. Microwave power transmission lines find application notably in plasma physics or, in thermonuclear fusion where it is often necessary to convey very high power values of electromagnetic energy, at millimetrical or infra-millimetrical wavelengths, over fairly large distances which may sometimes go to tens of meters, the transmission being done from microwave power generators to equipment using the power which may be a tokamak for example.
2. Description of the Prior Art
A known way of reducing transmission losses is to use oversized waveguides as transmission lines. Specific elements are used to deflect the direction of propagation of the electromagnetic energy within the transmission line (for example through elbows) or again to change its mode of propagation (for example, through mode converters, corrugated oversized guides, etc.).
In lower power transmission lines, there is a known method that uses a directional coupler (FIG. 1) to tap a part of the transmitted energy or, in reverse, to tap a part of the reflected energy. A calibration of the coupler makes it possible, by measuring the tapped energy, to compute the power conveyed by the transmission line itself.
A standard directional coupler includes a waveguide joined to the main waveguide of the transmission line, coupled to this main waveguide by a series of coupling holes distributed over a length L (FIG. 1). The standard coupler of FIG. 1 may be characterized by two parameters:
1) the coupling factor C=P.sub.3 /P.sub.1 and PA0 2) the directivity D=P.sub.3 /P.sub.4 measured at P.sub.2 =0 (zero reflected power)
where:
P.sub.1 =incident power in the main wave guide PA1 P.sub.2 =reflected power from the output in the main wave guide PA1 P.sub.3 =tapped incident power in the secondary wave guide PA1 P.sub.4 =tapped reflected power in the secondary wave guide
While it may be easy to make a standard coupler in the case of the rectangular main guides that work in fundamental propagation modes or have a low propagation value (e.g., the mode of propagation in a wave guide can be TE.sub.xy and the indices x,y have a low value), this is not the case for special guides used to convey the very high power required for thermonuclear fusion. Indeed, applications such as these often use oversized guides that are circular-sectioned, smooth or corrugated, possibly surrounded by a vacuum chamber or a coating that enables the walls of the guide to be cooled by a circulating liquid or gas to remove the heat generated in the conductive walls of the waveguide by microwave losses.
The structure of a microwave power transmission line formed by guides of this kind is complicated and hardly allows for the addition of a secondary guide to form a standard coupler as shown schematically in FIG. 1.
An object of the present invention is to overcome this difficulty by proposing a coupler with a novel design that does not have these drawbacks.
A very high power transmission line almost always has elbows, generally close to the generator or to the using equipment. This is for practical reasons of prime importance. For example, the generator is often of the gyrotron type, namely an electron tube that generally works in a vertical position and has its energy coming out of it vertically. To convey its energy to the equipment. Using this power, which may be positioned at a distance tens of meters away in a horizontal direction, at least one elbow would be necessary. Similarly, near the user equipment (a tokamak for example), a large number of microwave transmission lines converge from the different sources and have to be coupled to the user equipment. These transmission lines are positioned among other devices (such as probes, gas sources, various supplies, electro-magnets, cooling equipment, lasers etc. which may give rise to phenomena of mechanical interference. The phenomena of mechanical interference will then be circumvented by means of elbows placed in the transmission line close to the user equipment.
For high power values and high frequencies (and hence short wavelengths), mirror elbows are frequently used. These have a structure, two prior art examples of which are shown, schematically in FIGS. 2 and 3.